1. Technical Field
The present invention relates to a quartz crystal resonator in which a quartz crystal resonator element has a main vibrating portion integrally formed with a supporting portion and is covered with casings of glass or the like.
2. Related Art
The recent trend toward smaller and thinner electronic equipment requires smaller and thinner designs of quartz crystal resonators incorporated therein. For the purpose of reducing the size and thickness of a quartz crystal resonator, for example, Japanese Patent No. 3390348 and JP-A-2006-157369 have disclosed a structure of a quartz crystal resonator (which is called a chip size package (CSP) quartz crystal resonator).
Referring to FIG. 16, a description will be given of the above quartz crystal resonator. FIG. 16 is a front sectional view showing an AT-cut quartz crystal resonator using a quartz crystal substrate, as an example of the well-known quartz crystal resonator.
A quartz crystal resonator 500 is comprised of a quartz crystal resonator element 505 and first and second casings 506 and 507 as covers provided so as to sandwich and cover the quartz crystal resonator element 505. The quartz crystal resonator element 505 is constituted using a quartz crystal substrate (which is called an AT-cut quartz crystal substrate) having a plane including an X axis (electrical axis) as a crystallographic axis of a quartz crystal and a Z′ axis having an inclination of approximately 35.25 degrees from a Z axis (optical axis) around the X axis.
In the quartz crystal resonator element 505, a main vibrating portion 501, which has excitation electrodes 503 and 504 formed on upper and lower surfaces thereof, is integrally formed with a supporting portion 502 provided so as to surround the main vibrating portion 501.
The first and second casings 506 and 507 are, for example, made of versatile glass such as blue plate glass. The first and second casings 506 and 507 have peripherally-formed protrusions 508 and 509, respectively, at outer peripheries thereof. The first and second casings 506 and 507 are bonded to the supporting portion 502 at the protrusions 508 and 509 so as to sandwich the quartz crystal resonator element 505 therebetween.
However, in the above quartz crystal resonator 500, a thermal expansion coefficient of the quartz crystal resonator element 505 is different from that of the first and second casings 506 and 507. Accordingly, there is a variation in the amount of expansion caused due to changes in environmental temperature. For example, a thermal expansion coefficient in the X-axis direction of a quartz crystal is 19.7×10−6 per degrees centigrade, whereas that of blue plate glass is 8.1×10−6 per degrees centigrade.
Environmental temperature changes will cause a thermal stress because of a difference between the amounts of expansions at places where the supporting portion 502 has been bonded to each of the protrusions 508 and 509. The thermal stress affects a resonance frequency of the quartz crystal resonator 500 using the above-mentioned AT-cut crystal substrate. Therefore, there arises a so-called problem that frequency-temperature characteristics are deteriorated.